TIMEBASE CIRCUITS
Every picture-tube must have a means of deflecting its scanning spot to all parts of the screen or target. Except in oscilloscope displays the electron beam is deflected by magnetic fields in the neck of the tube. These are generated by deflection coils wound on a ferrite former and securely fitted to the tube neck. One pair of coils is used for vertical spot deflection, and between them make horizontal lines of magnetic flux in the tube neck: these deflect the beam vertically, in a direction determined by the polarity of the flux lines, and to a degree proportional to the intensity (or number of lines) of flux. For horizontal deflection of the electron beam a second pair of coils is used, this time generating vertical lines of force through the tube neck. Again beam deflection is proportional to strength and direction of the magnetic field. The complete assembly of coils and moulded ferrite former is called a scan yoke. Each yoke is manufactured specifically to match, physically and magnetically, the tube type with which it is intended to be used; this is particularly true of colour tubes, as was made clear in Chapter 5.
The intensity of magnetic field developed by an electromagnet is proportional to the current flowing in it. Since beam deflection is exactly proportional to field strength, the basic requirement for linear image scan is a sawtooth current waveform in each pair of deflection coils. Unless the coils behave as a pure resistor, the voltage appearing across them will not be in sawtooth form – in fact, to create a saw- tooth (linearly rising) current in a pure inductor, a constant d.c. volt- age must be applied to it.
At very small deflection angles such as those used in camcorder viewfinder tubes a linear deflecting current for line and field scans will result in the required constant velocity of the scanning spot over the target. In large flat-faced display tubes, especially 110° deflection types, a linear deflection characteristic will not impart constant scan- ning speed over the screen, however: the differing beam path lengths between screen centre and screen edges tends to speed up the ‘linear’ progress of the beam towards picture extremities, and correction must be made for this in the shape of the scanning current waveform passed through the deflection coils.
At field frequency (50 Hz) the scanning coils behave almost as a resistance during the forward stroke, so the required voltage drive approximates to a sawtooth, with compensatory shaping to correct for scan coil inductance and where applicable the ‘flat-face’ effect. Here the scan output stage acts in similar fashion to a conventional amplifier, e.g. an audio output stage. On the other hand, the line scan- coil pair represent almost pure inductance at the much higher (15625 Hz) horizontal scanning rate; to correctly drive these, then, a constant voltage in one direction must be applied for the 52 μs of the active picture period, then a higher constant voltage in the opposite direction for 12 μs to achieve a complete reversal of magnetic field and a complete traverse of the screen by the scanning spot on its flyback stroke.
Time bases, then, consist of three basic sections: a timing source (in practice some form of oscillator, locked to incoming sync pulses); a shaping stage, consisting of ramp generator for field applications or a pulse generator for line time bases; and a power output stage as a means of driving rapidly changing currents through the scan coils.